Any mob does not take damage when on wither roses

I loaded in a superflat world to check out some questions about Minecraft I had and put them to the test (Credits to xisumavoid). On my 10th question, I asked myself, "How do bees chose their favourite flower?" I was about to write, "Randomly" in my book and quill when I saw a bee collecting pollen on a wither rose. Aren't they supposed to take damage on wither roses? (Edit: I tried many mobs, all do not take damage on wither roses.)